Don’t need to wait, already wrote about it:

What is Certificate Pinning?

Certificate pinning is the mechanism of associating a domain name with an expected SSL/TLS certificate, technically and more accurately known as an X.509 certificate.

Whenever the user clicks on a link, the device needs to establish a connection with the server hosting that domain name, and for this to happen, a TLS handshake takes place in order that both parties can exchange messages, so that they can verify each other, establish the encryption algorithms to use, and finally to set the session keys to be used thereafter. During the TLS handshake, when the device receives the server certificate, it only establishes the connection if it trusts on that specific certificate, hence it is said that the connection is pinned.